1. Field of the Invention
This invention relates to improvements in a process for calcination of cement-clinkers.
2. Description of the Prior Art
As well known, at least two operations for calcination (decomposition) of limestone as a main raw material -- endothermic reaction -- and calcination of clinker (narrow sense) -- exothermic reaction -- are necessary in the production of cement-clinkers, and a major part of heat consumption amount in the calcination of cement (broad sense) is occupied by the said former calcination reaction including the heating of the raw materials.
Now, according to the cement calcination process to be carried out by means of a combination consisting of a suspension type raw material preheating apparatus and a rotary kiln, the cement raw material is preheated before being fed into the rotary kiln, while the calcination of the limestone in the raw material is simultaneously effected in some degree, and therefore it is true that a fairly good result can be attained in the said calcination process in the point of economization of combustion heat and of improvement of specific efficiency of rotary kiln.
In the said type of calcination process, however, the quantity of heat of the rotary kiln exhaust gas to be introduced into the raw material preheating apparatus is limited in relation to the temperature in the interior of the rotary kiln and to the problem on adhesion and growth of molten substances, and so, it is in general difficult to make the percentage of real calcination of the raw material in the said preheating apparatus about 40% or more, and the calcination of the remaining non-decomposed raw material of about 60% or more is to be effected in the rotary kiln or in a fluidized bed calcining device.
Accordingly, the calcination zone for effecting the supply of heat for the calcination reaction must inevitably be made long, and thus the size of the rotary kiln is necessarily made long thereby to prolong the retention time of raw material in the long rotary kiln. On these grounds, the said type of calcination process is defective and disadvantageous in that the operation is often interrupted or even made impossible due to various unfavorable phenomena occurring in accordance with the said prolonged retention time, such as increase of combustion heat, evaporation of alkali content, adhesion, growth and condensation of molten raw material in the flue and on the wall of apparatus.
In order to overcome such defect and disadvantage, a new type of calcination of cement-clinkers has been proposed where a calcining device having a separate heat source is combined with the rotary kiln or a fluidized bed calcining device to be used in place thereof and with the suspension type raw material preheating apparatus, and the limestone in the raw material is, after being almost completely calcined in the said calcining device, transported to the rotary kiln or to the fluidized bed calcining apparatus while kept at a high temperature.